Edge couplers in the back-end-of-line stack of a photonic chip having a sealed cavity

ABSTRACT

According to an aspect of the present disclosure, an edge coupler for a photonic chip is provided. The edge coupler includes a substrate having a top surface, a sealed cavity in the substrate, a waveguide core, and a back-end-of-line stack. The sealed cavity has varying depths relative to the top surface of the substrate. The waveguide core is over the sealed cavity. The back-end-of-line stack includes a side edge, an interlayer dielectric layer, and an assisting waveguide. The assisting waveguide is on the interlayer dielectric layer adjacent to the side edge. The assisting waveguide and the waveguide core have an overlapping arrangement with the sealed cavity in the substrate.

TECHNICAL FIELD

The present disclosure relates generally to photonic chips, and moreparticularly to edge couplers and methods of forming the same.

BACKGROUND

Photonic chips integrate optical components and electronic componentsinto a single platform. The optical components may include, for example,waveguides, optical switches, couplers, and modulators. The electroniccomponents may include, for example, field-effect transistors. Theelectronic components may be operatively coupled to the opticalcomponents for the functioning of the photonic chips.

An edge coupler is commonly used for coupling light of a given mode froma light source, such as a laser or an optical fiber, to the otheroptical components and/or electronic components on the photonic chip. Asthe light source is typically larger than the edge coupler, the edgecoupler may be unable to fully confine the incident mode received and asa result, the edge coupler may exhibit significant power losses.

Therefore, solutions are provided to overcome, or at least ameliorate,the disadvantages described above.

SUMMARY

To achieve the foregoing and other aspects of the present disclosure,edge couplers and methods of forming the same are presented.

According to an aspect of the present disclosure, an edge coupler for aphotonic chip is provided. The edge coupler includes a substrate havinga top surface, a sealed cavity in the substrate, a waveguide core, and aback-end-of-line stack. The sealed cavity has varying depths relative tothe top surface of the substrate. The waveguide core is over the sealedcavity. The back-end-of-line stack includes a side edge, an interlayerdielectric layer, and an assisting waveguide. The assisting waveguide ison the interlayer dielectric layer adjacent to the side edge. Theassisting waveguide and the waveguide core have an overlappingarrangement with the sealed cavity in the substrate.

According to another aspect of the present disclosure, an edge couplerfor a photonic chip is provided. The edge coupler includes a substratehaving a bottom surface, a sealed cavity in the substrate, a waveguidecore, and a back-end-of-line stack. The substrate has varying heightsrelative to the bottom surface. The waveguide core is over thesubstrate. The back-end-of-line stack includes a side edge, aninterlayer dielectric layer, and a first assisting waveguide. The firstassisting waveguide is on the interlayer dielectric layer adjacent tothe side edge. The first assisting waveguide and the waveguide core havean overlapping arrangement with the sealed cavity in the substrate.

According to yet another aspect of the present disclosure, a method offorming an edge coupler for a photonic chip is provided. The methodincludes forming a sealed cavity in a substrate and forming a waveguidecore over the sealed cavity. The cavity is collectively surrounded by aninsulator layer and a base substrate of the substrate. Aback-end-of-line stack is formed over the sealed cavity and includesforming an interlayer dielectric layer over the waveguide core andforming an assisting waveguide over the interlayer dielectric layer. Aside edge of the interlayer dielectric layer and an end surface of theassisting waveguide form a part of a side edge of the back-end-of-linestack. The assisting waveguide and the waveguide core have anoverlapping arrangement with the sealed cavity in the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present disclosure will be better understood froma reading of the following detailed description, taken in conjunctionwith the accompanying drawings:

FIGS. 1A and 1B are exemplary top-down views of an edge coupler of aphotonic chip at an initial fabrication stage of a processing method,according to embodiments of the disclosure.

FIGS. 2A through 2D are cross-sectional views of the edge coupler takengenerally along respective lines 2A-2A, 2B-2B, 2C-2C, and 2D-2D in FIG.1A, according to embodiments of the disclosure.

FIGS. 3A and 3B are exemplary top-down views of the edge coupler at afabrication stage subsequent to FIG. 1A, according to embodiments of thedisclosure.

FIGS. 4A through 4D are cross-sectional views of the edge coupler takengenerally along respective lines 4A-4A, 4B-4B, 4C-4C, and 4D-4D in FIG.3A, according to embodiments of the disclosure.

FIGS. 5A through 5C are exemplary top-down views of the edge coupler ata fabrication stage subsequent to FIG. 3A, according to embodiments ofthe disclosure.

FIGS. 6A through 6D are cross-sectional views of the edge coupler takengenerally along respective lines 6A-6A, 6B-6B, 6C-6C, 6D-6D in FIG. 5A,according to embodiments of the disclosure.

For simplicity and clarity of illustration, the drawings illustrate thegeneral manner of construction, and certain descriptions and details ofwell-known features and techniques may be omitted to avoid unnecessarilyobscuring the discussion of the described embodiments of the disclosure.

Additionally, features in the drawings are not necessarily drawn toscale. For example, the dimensions of some of the features in thedrawings may be exaggerated relative to other features to help improveunderstanding of embodiments of the device. The same reference numeralsin different drawings denote the same features, while similar referencenumerals may, but do not necessarily, denote similar features.

DETAILED DESCRIPTION

The present disclosure relates generally to photonic chips, and moreparticularly to edge couplers and methods of forming the same. Variousembodiments of the present disclosure are now described in detail withaccompanying drawings. It is noted that like and corresponding featuresare referred to by the use of the same reference numerals. Theembodiments disclosed herein are exemplary, and not intended to beexhaustive or limiting to the disclosure.

FIGS. 1A and 1B are exemplary top-down views of an edge coupler 100 atan initial fabrication stage of a processing method, according toembodiments of the disclosure. FIGS. 2A through 2D are cross-sectionalviews of the edge coupler 100 taken generally along respective lines2A-2A, 2B-2B, 2C-2C, and 2D-2D in FIG. 1A, according to embodiments ofthe disclosure. The edge coupler 100 may be used in a photonic chip.

The edge coupler 100 may have a longitudinal axis 100L, a transverseaxis 100T, and a vertical axis 100V. The longitudinal axis 100L, thetransverse axis 100T, and the vertical axis 100V determine the length,the width, and the height, respectively, of the edge coupler 100. Theedge coupler 100 may include a side edge 102. The side edge 102 of theedge coupler 100 may be arranged substantially perpendicular to thelongitudinal axis 100L. In an embodiment of the disclosure, the sideedge 102 of the edge coupler 100 may be substantially planar. In anotherembodiment of the disclosure, the side edge 102 of the edge coupler 100may serve as an input from a light source, such as a laser or an opticalfiber, to the other optical and/or electronic components in the photonicchip.

The edge coupler 100 may include a substrate 104. The substrate 104 mayinclude a layered substrate, such as a semiconductor-on-insulator (SOI)substrate, or a bulk substrate. For purposes of description in thepresent disclosure, the substrate 104 will be referred to as a layeredsubstrate. The substrate 104 may include a buried insulator layer 106arranged between a device layer (not shown) and a base substrate 108.The device layer may be where the optical components, including the edgecoupler 100, and the electronic components of the photonic chip may bearranged adjacent to, in, and/or above the device layer.

The buried insulator layer 106 may include a side edge 110, and the sideedge 110 of the buried insulator layer 106 may form a part of the sideedge 102 of the edge coupler 100. The buried insulator layer 106 mayinclude a dielectric material, for example, silicon dioxide. In anembodiment of the disclosure, the side edge 110 of the buried insulatorlayer 106 may be substantially planar.

The base substrate 108 may have a top surface 112 upon which the buriedinsulator layer 106 overlies and a bottom surface 114 laterally oppositethe top surface 112. The base substrate 108 may include a semiconductormaterial, such as silicon, silicon germanium, silicon carbide, or othersemiconductor compounds, for example, II-VI or III-V semiconductorcompounds.

As illustrated in FIGS. 2A through 2C, the base substrate 108 mayinclude a cavity 116 therewithin and under the buried insulator layer106; the outline of the cavity 116 in FIGS. 1A and 1B arediagrammatically shown by a long dash-short-dash line for purposes ofillustration. The cavity 116 may have a finite length L_(C) which liessubstantially parallel to the longitudinal axis 100L of the edge coupler100 and a width We which lies perpendicular thereto. The cavity 116 maybe collectively surrounded by the buried insulator layer 106 from aboveand by the base substrate 108 from below and laterally, which fullyseals the cavity 116. For example, portions of the base substrate 108may enclose the cavity 116 and define the longitudinal and transverseboundaries of the cavity 116. In another example, a portion of theburied insulator layer 106 may form a substantially flat ceiling of thecavity 116.

The cavity 116 may have varying depths relative to the top surface 112of the base substrate 108. For example, as illustrated in FIG. 2A, thecavity 116 may include a central cavity portion 116 c that may bearranged substantially parallel to the longitudinal axis 100L of theedge coupler 100. The central cavity portion 116 c may include ends 118that determine the length L_(C) of the cavity 116. The central cavityportion 116 c between the ends 118 may have varying depths. For example,the central cavity portion 116 c may include a center region 116 c ₁arranged between an end region 116 c ₂ and an end region 116 c ₃; theend region 116 c ₂ may be arranged proximate to the side edge 102 of theedge coupler 100. The center region 116 c ₁ of the cavity 116 may have adepth D1 and each end region 116 c ₂, 116 c ₃ may have a depth D2 andD3, respectively, which is shallower than the depth D1 of the centerregion 116 c ₁.

The varying depths of the cavity 116 may minimize potential opticalsignal losses through the substrate 104 when the optical signalspropagate through the edge coupler 100 and may take on variousconfigurations. For example, the respective depths D2, D3 of the endregions 116 c ₂, 116 c ₃ may be substantially equal, and the centralcavity portion 116 c may acquire a “T-shaped” profile, as illustrated inFIG. 2A. In another example, the end regions 116 c ₂, 116 c ₃ may notnecessarily have a substantially equal depth, as long as the depth D1 ofthe center region 116 c ₁ is the deepest relative to that of the endregions 116 c ₂, 116 c ₃. In yet another example, the depth D3 of theend region 116 c ₃ and the depth D1 of the center region 116 c ₁ mayhave a substantially equal depth, and the depth D2 of the end region 116c ₂ may be shallower thereto.

The center region 116 c ₁ and the respective end region 116 c ₂, 116 c ₃may each have a bottom surface 120 c ₁, 120 c ₂, 120 c ₃, respectively.In an embodiment of the disclosure, the bottom surfaces 120 c ₁, 120 c₂, 120 c ₃ of the respective center region 116 c ₁, the end region 116 c₂, and the end region 116 c ₃ may be substantially planar. In anembodiment of the disclosure, the bottom surface 120 c ₁ of the centerregion 116 c ₁ may be arranged at a depth level lower than therespective bottom surfaces 120 c ₂, 120 c ₃, of the end regions 116 c ₂,116 c ₃. In another embodiment of the disclosure, the center region 116c ₁ of the central cavity portion 116 c may have a length that is longerthan that of each end region 116 c ₂, 116 c ₃.

The cavity 116 may further include side cavity portions 116 s ₁, 116 s ₂arranged symmetrically relative to the central cavity portion 116 c. Asillustrated in FIGS. 2B and 2C, the side cavity portions 116 s ₁, 116 s₂ may be arranged at laterally opposing sides of the central cavityportion 116 c and may extend along with the longitudinal axis 100L ofthe edge coupler 100. The side cavity portions 116 s ₁, 116 s ₂ may beconnected to and merged with the central cavity portion 116 c. The sidecavity portions 116 s ₁, 116 s ₂ may include curved surfaces 122. Thecurved surfaces 122 of the side cavity portions 116 s ₁, 116 s ₂ mayintersect the bottom surfaces 120 c ₁, 120 c ₂, 120 c ₃ of the centralcavity portion 116 c. A base substrate ridge 124 may be centered betweenthe side cavity portions 116 s ₁, 116 s ₂ and may be arranged directlyunder the central cavity portion 116 c. The base substrate ridge 124 mayextend along with the longitudinal axis 100L of the edge coupler 100 andmay have a length that is no longer than the length of the centralcavity portion 116 c.

FIG. 2B illustrates the side cavity portions 116 s ₁, 116 s ₂ arrangedat laterally opposing sides of the central cavity portion 116 c at thecenter region 116 c ₁. The side cavity portions 116 s ₁, 116 s ₂ mayhave a maximum depth D4 that is deeper than the depth D1 of the centerregion 116 c ₁ of the central cavity portion 116 c. FIG. 2C illustratesthe side cavity portions 116 s ₁, 116 s ₂ arranged at laterally opposingsides of the central cavity portion 116 c at the end region 116 c ₃. Theside cavity portions 116 s ₁, 116 s ₂ may have a maximum depth D5 thatis deeper than the depth D3 of the end region 116 c ₃ of the centralcavity portion 116 c. The maximum depth D5 of the side cavity portions116 s ₁, 116 s ₂ at the end region 116 c ₃ of the central cavity portion116 c may be shallower than the maximum depth D4 of the side cavityportions 116 s ₁, 116 s ₂ at the center region 116 c ₁.

Accordingly, the base substrate ridge 124 may have varying heightsrelative to the bottom surface 114 of the base substrate 108. Forexample, the base substrate ridge 124 under the center region 116 c ₁ ofthe central cavity portion 116 c may have a height H1 that is shorterthan that of the height H2 of the base substrate ridge 124 under the endregion 116 c ₃ of the central cavity portion 116 c.

The cavity 116 may be fully sealed to define an air gap, and the air gapmay contain air at or near atmospheric pressure, or at sub-atmosphericpressure, for example, a partial vacuum. Although referred to as an “airgap”, the elemental composition of the air gap can include differentgases and should not be construed as having any particular elementalcomposition, for example, any number and type of gases may be present inthe air gap defined by the sealed cavity 116.

The air gap defined by the sealed cavity 116 may be characterized by apermittivity or a dielectric constant of near unity, for example, vacuumpermittivity. The permittivity of the air gap may be less than thedielectric constant of the surrounding solid material, such as theburied insulator layer 106.

The cavity 116 in the substrate 104 may be formed by the followingexemplary fabrication process. Openings (not shown) may be formedthrough the buried insulator layer 106 using a patterning technique,including lithography and etching processes. The openings may expose aportion of the base substrate 108 therein. The openings may be elongatedor slotted in shape and may be arranged with a given pitch in parallelrows that are symmetrically arranged relative to the longitudinal axis100L of the edge coupler 100. The openings may be substantially alignedat a location above the base substrate 108 where the deepest depth ofeach side cavity portion 116 s ₁ and 116 s ₂ may be desired.

The openings may define pilot holes for performing a subsequent materialremoval technique on the base substrate 108. The openings may provideaccess to the base substrate 108 for the material removal techniqueperformed to form the cavity 116. The material removal technique mayinclude a lateral removal component that deepens the cavity 116 and avertical removal component that widens the cavity 116. The verticalremoval component of the material removal technique may merge the sidecavity portions 116 s ₁, 116 s ₂ to form the central cavity portion 116c. In an embodiment of the disclosure, the material removal techniquemay include a dry isotropic etching process, including a reactive ionetching process. In another embodiment of the disclosure, the materialremoval technique may include a wet isotropic etching process, such asone including liquid etchants.

The edge coupler 100 may further include a dielectric layer 126 arrangedover the substrate 104. Specifically, the dielectric layer 126 mayoverlie the buried insulator layer 106 of the substrate 104. Thedielectric layer 126 may include a side edge 128, and the side edge 128of the dielectric layer 126 may form a part of the side edge 102 of theedge coupler 100. The dielectric layer 126 may include a dielectricmaterial, for example, doped or undoped silicon dioxide. The dielectriclayer 126 may be deposited over the substrate 104 using a depositiontechnique, including a chemical vapor deposition process, duringmiddle-of-line processing. In an embodiment of the disclosure, the sideedge 128 of the dielectric layer 126 may be substantially planar. Inanother embodiment of the disclosure, the side edge 128 of thedielectric layer 126 may be substantially planar with the side edge 110of the buried insulator layer 106.

The edge coupler 100 may yet further include a waveguide core 130. Thewaveguide core 130 may include a front section 130 a, a middle section130 b, and an end section 130 c. The middle section 130 b of thewaveguide core 130 may be arranged between the front section 130 a andthe end section 130 c; the transition of the front section 130 a to themiddle section 130 b and the transition of the middle section 130 b tothe end section 130 c are diagrammatically shown by dotted lines forpurposes of illustration. The waveguide core 130 may have a longitudinalaxis 130L, and the front section 130 a, the middle section 130 b, andthe end section 130 c may be aligned with, and extend along, thelongitudinal axis 130L of the waveguide core 130. In an embodiment ofthe disclosure, the longitudinal axis 130L of the waveguide core 130 maybe arranged substantially parallel to the longitudinal axis 100L of theedge coupler 100.

The waveguide core 130 may include end surfaces 132, 134 that terminatethe front section 130 a and the end section 130 c thereof. The endsurfaces 132, 134 may determine a finite length of the waveguide core130 therebetween. In an embodiment of the disclosure, the respectivelengths of the front section 130 a and the end section 130 c may besubstantially equal to each other. In another embodiment of thedisclosure, the length of the middle section 130 b may be longer thanthe respective lengths of the front section 130 a and the end section130 c.

The front section 130 a of the waveguide core 130 may be arrangedproximate to the side edge 102 of the edge coupler 100 and the endsurface 132 of the waveguide core 130 may be spaced apart therefrom. Thefront section 130 a of the waveguide core 130 may serve as an input tothe waveguide core 130 for the propagation of the light source throughthe waveguide core 130 and the end section 130 c may serve as an outputfor the propagation of the light source from the edge coupler 100 toother components in the photonic chip. Accordingly, the end section 130c of the waveguide core 130 may be arranged proximate to the othercomponents, for example, optical components, such as a Mach-Zehndermodulator, and/or electronic components, such as a field-effecttransistor, in the photonic chip. In an embodiment of the disclosure,the end surfaces 132, 134 of the waveguide core 130 may be substantiallyplanar and may be substantially parallel to each other.

The front section 130 a and the end section 130 c of the waveguide core130 may have a substantially constant width dimension along theirrespective lengths, though not necessarily the same width dimension. Inan embodiment of the disclosure, the width of the front section 130 amay be narrower than the width of the end section 130 c, as illustratedin FIGS. 1A and 1B.

The middle section 130 b of the waveguide core 130 may have a widthdimension that varies with position along its length. For example, thewaveguide core 130 may have a minimum width that may be substantiallyequal to the width of the front section 130 a and a maximum width thatmay be substantially equal to the width of the end section 130 c, andthe middle section 130 b may be tapered (i.e., narrows) in a directiontowards the end surface 132. In another example, the width of the middlesection 130 b may increase with increasing distance from the end surface132 of the waveguide core 130. In an embodiment of the disclosure, thewidth of the middle section 130 b may vary along the longitudinal axis130L based on a linear function to provide a trapezoidal shape. Inanother embodiment of the disclosure, the width dimension of the middlesection 130 b may vary along the longitudinal axis 130L based on anon-linear function, such as a quadratic, parabolic, or exponentialfunction.

The waveguide core 130 may be arranged directly and primarily above thecavity 116 in the substrate 104. In an embodiment of the disclosure, thewaveguide core 130 may be arranged directly above the central cavityportion 116 c of the cavity 116. For example, the front section 130 a ofthe waveguide core 130 may be arranged directly above the center region116 c ₁ of the central cavity portion 116 c, the middle section 130 bmay traverse across the center region 116 c ₁, and the end region 116 c₃ of the central cavity portion 116 c, and the end section 130 c of thewaveguide core 130 may be arranged at an offset from the cavity 116 suchthat the end section 130 c does not overlie the cavity 116. In anotherembodiment of the disclosure, at least a portion of the front section130 a of the waveguide core 130 may be arranged directly above the endregion 116 c ₂ of the central cavity portion 116 c.

The waveguide core 130 may include a dielectric material, for example,silicon nitride. The waveguide core 130 may be formed by a depositiontechnique, including a chemical vapor deposition process, andsubsequently patterned by a patterning technique, including lithographyand etching processes, during middle-of-line processing.

Additionally, the edge coupler 100 may include assisting waveguides 136,138 arranged on the dielectric layer 126 at the same level as thewaveguide core 130 with a spaced arrangement. The assisting waveguides136, 138 may be further fully arranged above the cavity 116 in thesubstrate 104. For example, the assisting waveguides 136, 138 may bearranged above each side cavity portion 116 s ₁, 116 s ₂ of the cavity116, respectively, as illustrated in FIG. 2B. In another example, thecenter region 116 c ₁ and the end region 116 c ₃ of the central cavityportion 116 c may be arranged between the assisting waveguides 136, 138.

The assisting waveguides 136, 138 may extend along with the longitudinalaxis 100L of the edge coupler 100 and may be substantially parallel tothe longitudinal axis 130L of the waveguide core 130. The assistingwaveguides 136, 138 may be elongated or slotted in shape. For example,each assisting waveguide 136, 138 may have a length and a width; thelength dimension may be significantly larger than the width dimension.Each assisting waveguide 136, 138 may have a finite length and may beterminated by an end surface 140 and an end surface 142 arrangedlaterally opposite the end surface 140, and the end surfaces 140, 142determine the length therebetween. In an embodiment of the disclosure,the end surfaces 140, 142 of the assisting waveguides 136, 138 may besubstantially planar and may be substantially parallel to each other. Inanother embodiment of the disclosure, the assisting waveguides 136, 138may have a substantially constant width dimension along their respectivelengths. In yet another embodiment of the disclosure, the assistingwaveguides 136, 138 may have a substantially equal width dimension.

The end surfaces 140 of the assisting waveguides 136, 138 may be spacedapart from the side edge 102 of the edge coupler 100, and the length ofeach assisting waveguide 136, 138 may be shorter than the length of thewaveguide core 130. For example, the assisting waveguides 136, 138 mayhave a length at most as long as the length of the waveguide core 130that is arranged over the center region 116 c ₁ of the central cavityportion 116 c. In another example, the end surfaces 140 of the assistingwaveguides 136, 138 may be substantially coplanar with the end surface132 of the waveguide core 130, and the end surfaces 142 of the assistingwaveguides may terminate before the end section 130 c of the waveguidecore 130. In an embodiment of the disclosure, the end surfaces 142 ofthe assisting waveguides 136, 138 may terminate at a plane thatintersects with the middle section 130 b of the waveguide core 130. Asillustrated in FIG. 1A, the end surfaces 142 of the assisting waveguides136, 138 may terminate at the plane that intersects approximately themidpoint of the middle section 130 b of the waveguide core 130.Alternatively, the end surfaces 142 of the assisting waveguides 136, 138may terminate before the middle section 130 b of the waveguide core 130,as illustrated in FIG. 1B.

As illustrated in FIGS. 1A, 1B, and 2B, the assisting waveguides 136,138 may be arranged at opposing sides of the waveguide core 130. In anembodiment of the disclosure, the assisting waveguides 136, 138 may bearranged symmetrically relative to the waveguide core 130. Even thoughFIGS. 1A and 1B illustrate the assisting waveguides 136, 138 as beingsubstantially identical to each other, the assisting waveguides 136, 138may adopt different geometric configurations from each other, withoutdeparting from the scope and spirit of the present disclosure. In anembodiment of the disclosure, the assisting waveguides 136, 138 may havea different length dimension from each other. For example, one of theassisting waveguides 136/138 may have a similar length dimension as theassisting waveguides 136, 138 in FIG. 1A, while the other assistingwaveguide 138/136 may have a similar length dimension as the assistingwaveguides 136, 138 in FIG. 1B. In another embodiment of the disclosure,the assisting waveguides 136, 138 may have a different width dimensionfrom each other.

The assisting waveguides 136, 138 may be formed concurrently with thewaveguide core 130 and may include a dielectric material that is similarto the waveguide core 130 for ease of fabrication. For example, theassisting waveguides 136, 138 may include silicon nitride.Alternatively, the assisting waveguides 136, 138 may include adielectric material that is different from the dielectric material ofthe waveguide core 130, for example, silicon carbide.

FIGS. 3A and 3B are exemplary top-down views of the edge coupler 100 ata fabrication stage subsequent to FIG. 1A, according to embodiments ofthe disclosure. FIGS. 4A through 4D are cross-sectional views of theedge coupler 100 taken generally along respective lines 4A-4A, 4B-4B,4C-4C, and 4D-D in FIG. 3A, according to embodiments of the disclosure.A dielectric stack 144 may be arranged over the dielectric layer 126,the waveguide core 130, and the assisting waveguides 136, 138.

The dielectric stack 144 may include a side edge 146, and the side edge146 of the dielectric stack 144 may form a part of the side edge 102 ofthe edge coupler 100. The side edge 146 of the dielectric stack 144 maybe further arranged over the side edge 128 of the dielectric layer 126and the side edge 110 of the buried insulator layer 106. In anembodiment of the disclosure, the side edge 146 of the dielectric stack144 may be substantially planar. In another embodiment of thedisclosure, the side edge 146 of the dielectric stack 144 may besubstantially coplanar with the side edge 128 of the dielectric layer126 and the side edge 110 of the buried insulator layer 106.

The dielectric stack 144 may include one or more layers of dielectricmaterials, for example, silicon dioxide and/or a dielectric materialhaving a dielectric constant lower than that of silicon dioxide.Examples of dielectric materials having a dielectric constant lower thanthat of silicon dioxide include carbon-doped silicon dioxide, tetraethylorthosilicate (TEOS), borophosphosilicate glass (BPSG), or undopedsilicate glass (USG). The dielectric stack may also include fluorinateddielectric materials, for example, fluorinated silicon dioxide orfluorinated TEOS. The dielectric stack 144 may further include asemiconductor material, for example, amorphous silicon. The dielectricstack 144 may be deposited using a deposition technique, including achemical vapor deposition process, during middle-of-line processing.

Another dielectric stack 148 may be arranged over the dielectric stack144. Similar to the dielectric stack 144, the dielectric stack 148 mayinclude one or more layers of dielectric materials and/or fluorinateddielectric materials, though not necessarily the same materials. In anembodiment of the disclosure, each layer in the dielectric stack 148 maybe referred to as an interlayer dielectric layer. The dielectric stack148 may be deposited using a deposition technique, including a chemicalvapor deposition process, during back-end-of-line processing.

The dielectric stack 148 may include a side edge 150, and the side edge150 of the dielectric stack 148 may form a part of the side edge 102 ofthe edge coupler 100. The side edge 150 of the dielectric stack 148 maybe further arranged over the side edge 146 of the dielectric stack 144,the side edge 128 of the dielectric layer 126, and the side edge 110 ofthe buried insulator layer 106. In an embodiment of the disclosure, theside edge 150 of the dielectric stack 148 may be substantially planar.In another embodiment of the disclosure, the side edge 150 of thedielectric stack 148 may be substantially coplanar with the side edge146 of the dielectric stack 144, the side edge 128 of the dielectriclayer 126, and the side edge 110 of the buried insulator layer 106.

Assisting waveguides 152, 154, 156, 158 may be arranged on thedielectric stack 148 with a spaced arrangement. The assisting waveguide154 may be arranged laterally adjacent to the assisting waveguide 152,the assisting waveguide 156 may be arranged laterally adjacent to theassisting waveguide 154, and the assisting waveguide 158 may be arrangedlaterally adjacent to the assisting waveguide 156.

The assisting waveguides 152, 154, 156, 158 may extend along with thelongitudinal axis 100L of the edge coupler 100. For example, theassisting waveguide 156 may have a longitudinal axis 156L that may besubstantially parallel to the longitudinal axis 100L of the edge coupler100. Each of the assisting waveguides 152, 154, 156, 158 may beterminated by an end surface 160 and an end surface 162 arrangedlaterally opposite the end surface 160.

The end surfaces 160 of the assisting waveguides 154, 156 may bearranged over the side edge 150 of the dielectric stack 148, the sideedge 146 of the dielectric stack 144, the side edge 128 of thedielectric layer, and the side edge 110 of the buried insulator layer106, and may form a part of the side edge 102 of the edge coupler 100.In an embodiment of the disclosure, the end surfaces 160 of theassisting waveguides 154, 156 may be substantially planar. In anotherembodiment of the disclosure, the end surfaces 160 of the assistingwaveguides 154, 156 may be substantially coplanar with the side edge 150of the dielectric stack 148, the side edge 146 of the dielectric stack144, the side edge 128 of the dielectric layer, and the side edge 110 ofthe buried insulator layer 106.

The end surfaces 162 of the assisting waveguides 154, 156 may terminateat a plane that intersects with the waveguide core 130. In an embodimentof the disclosure, the end surfaces 162 of the assisting waveguides 154,156 may be substantially planar and substantially parallel to theirrespective end surfaces 160. In another embodiment of the disclosure,the end surfaces 162 of the assisting waveguides 154, 156 may besubstantially coplanar with each other. In yet another embodiment of thedisclosure, the end surfaces 162 of the assisting waveguides 154, 156may be substantially coplanar with the end surfaces 142 of theunderlying assisting waveguides 136, 138, as illustrated in FIG. 3A.

The end surfaces 160 of the assisting waveguides 152, 158 may bearranged spaced apart from the side edge 102 of the edge coupler 100 andmay not form a part of the side edge 102 of the edge coupler 100. Forexample, the end surfaces 160 of the assisting waveguides 152, 158 maybe arranged at an offset from the end surfaces 160 of the assistingwaveguides 154, 156. For example, the end surfaces 160 of the assistingwaveguides 152, 158 may terminate at a plane that intersects with theassisting waveguides 154, 156. In an embodiment of the disclosure, theend surfaces 160 of the assisting waveguides 152, 158 may besubstantially planar. In another embodiment of the disclosure, the endsurfaces 160 of the assisting waveguides 152, 158 may be substantiallycoplanar with each other.

The end surfaces 162 of the assisting waveguides 152, 158 may terminateat a plane that intersects with the waveguide core 130. In an embodimentof the disclosure, the end surfaces 162 of the assisting waveguides 152,158 may be substantially planar and substantially parallel to theirrespective end surfaces 160. In another embodiment of the disclosure,the end surfaces 162 of the assisting waveguides 152, 158 may besubstantially coplanar with each other. In yet another embodiment of thedisclosure, the end surfaces 162 of the assisting waveguides 152, 158may be substantially coplanar with the end surfaces 142 of theunderlying assisting waveguides 136, 138, as illustrated in FIG. 3A.

The end surfaces 160, 162 determine the lengths of the assistingwaveguides 152, 154, 156, 158, though the assisting waveguides 152, 154,156, 158 may not necessarily have the same length dimension. Forexample, the assisting waveguides 154, 156 may have a substantiallyequal length, and the assisting waveguides 152, 158 may have asubstantially equal length that may be shorter than the assistingwaveguides 154, 156. In an embodiment of the disclosure, the lengths ofthe assisting waveguides 152, 158 may have a length dimension that issubstantially equal to the respective lengths of the underlyingassisting waveguides 136, 138. In another embodiment of the disclosure,the assisting waveguides 154, 156 may have a length dimension that issubstantially equal to the length of the waveguide core 130.

The assisting waveguides 152, 154, 156, 158 may be arranged at an offsetover the assisting waveguides 136, 138, and the waveguide core 130, asillustrated in FIGS. 3A and 3B; the outlines of the assisting waveguides136, 138, and the waveguide core 130 are diagrammatically shown as dashlines, respectively. For example, as illustrated in FIG. 3A, theassisting waveguides 152, 154 may be arranged at laterally opposingsides of the underlying assisting waveguide 136, and the assistingwaveguides 156, 158 may be arranged at laterally opposing sides of theunderlying assisting waveguide 138. In another example, the assistingwaveguides 154, 156 may be arranged at laterally opposing sides of theunderlying waveguide core 130.

The assisting waveguides 152, 154, 156, 158 may be further arrangedsymmetrically relative to the underlying waveguide core 130. Forexample, the assisting waveguides 152, 154 may be arranged adjacent toone side of the waveguide core 130, and the assisting waveguides 156,158 may be arranged adjacent to the other side of the waveguide core130.

As illustrated in FIG. 3A, the assisting waveguides 152, 154, 156, 158may be arranged substantially parallel to each other. In an embodimentof the disclosure, the assisting waveguides 152, 154, 156, 158 may bearranged substantially parallel to the underlying waveguide core 130.

Alternatively, as illustrated in FIG. 3B, the assisting waveguides 152,158 may be shaped as a bend that curves away from the longitudinal axis100L of the edge coupler 100 along the transverse axis 100T. Forexample, a separation between the assisting waveguide 152 and theassisting waveguide 154 may increase with increasing distance from theirrespective end surfaces 162 and a separation between the assistingwaveguide 158 and the assisting waveguide 156 may increase withincreasing distance from their respective end surfaces 162. Theassisting waveguide 152 may curve away in an opposite direction relativeto the assisting waveguide 158.

In another example, the assisting waveguides 152, 158 may extend betweentheir respective end surfaces 162 over a curved or curvilinear path andmay have a given curvature over their respective curved arc length. Thecurvature of the assisting waveguide 152 may be reversed or invertedrelative to the curvature of the assisting waveguide 158. Accordingly,the assisting waveguides 152, 158 may trace smooth curves having acontinuously-turning tangent at their inner and outer radii over theirrespective lengths. In an embodiment of the disclosure, the assistingwaveguides 152, 158 may have substantially equal lengths.

The assisting waveguides 154, 156 may have a width dimension that varieswith position along its length. Referring to the assisting waveguide156, the assisting waveguide 156 may include a front section 156 a, amiddle section 156 b, and an end section 156 c. The middle section 156 bof the assisting waveguide 156 may be arranged between the front section156 a and the end section 156 c; the transition of the front section 156a to the middle section 156 b and the transition of the middle section156 b to the end section 156 c are diagrammatically shown by dottedlines for purposes of illustration. The assisting waveguide 156 may havea longitudinal axis 156L, and the front section 156 a, the middlesection 156 b, and the end section 156 c may be aligned with thelongitudinal axis 156L thereof. In an embodiment of the disclosure, thelongitudinal axis 156L of the assisting waveguide 156 may be arrangedsubstantially parallel to the longitudinal axis 130L of the waveguidecore 130.

The front section 156 a of the assisting waveguide 156 may have a widthdimension that varies with position along its longitudinal axis 156L.For example, the front section 156 a may be tapered (i.e., narrows) in adirection towards the end surface 160. In another example, the widthdimension of the front section 156 a may increase with increasingdistance from the end surface 160. In an embodiment of the disclosure,the width dimension of the front section 156 a may vary along thelongitudinal axis 156L based on a linear function to provide atrapezoidal shape. In another embodiment of the disclosure, the widthdimension of the front section 156 a may vary along the longitudinalaxis 156L based on a non-linear function, such as a quadratic,parabolic, or exponential function.

Similarly, the end section 156 c of the assisting waveguide 156 may alsohave a width dimension that varies with position along the longitudinalaxis 156L. For example, the end section 156 c may be tapered (i.e.,narrows) in a direction towards the end surface 162. In another example,the width dimension of the end section 156 c may increase withincreasing distance from the end surface 162. In an embodiment of thedisclosure, the width dimension of the end section 156 c may vary alongthe longitudinal axis 156L based on a linear function to provide atrapezoidal shape. In another embodiment of the disclosure, the widthdimension of the end section 156 c may vary along the longitudinal axis156L based on a non-linear function, such as a quadratic, parabolic, orexponential function. In yet another embodiment of the disclosure, theend section 156 c may be shorter than the front section 156 a of theassisting waveguide.

The middle section 156 b may have a substantially constant widthdimension, and the width of the middle section 156 b may besubstantially equal to the maximum width dimension of the front section156 a and the end section 156 c. In an alternative embodiment of thedisclosure, the assisting waveguide 156 may be modified to omit themiddle section 156 b while retaining the front section 156 a and the endsection 156 c. The end section 156 c may be lengthened to connect withthe front section 156 a in order to maintain the finite lengths of theassisting waveguide 156. In an embodiment of the disclosure, theassisting waveguide 154 may be substantially similar to the assistingwaveguide 156.

At least a portion of the assisting waveguides 154, 156 may be arrangeddirectly and primarily above the cavity 116 in the substrate 104. Forexample, the assisting waveguides 154, 156 may be arranged above eachside cavity portion 116 s ₁, 116 s ₂ of the cavity 116, respectively, asillustrated in FIGS. 4C and 4D. The assisting waveguides 154, 156 may bearranged into and out of the page of the accompanying drawings, and theassisting waveguide 156 is illustrated to be arranged into the page ofthe accompanying drawings and the outline of the assisting waveguide 156is diagrammatically shown with a double dash-dot line for purposes ofillustration. In an embodiment of the disclosure, a portion of the frontsection 156 a of the assisting waveguide 156 may be arranged directlyabove the base substrate 108, as illustrated in FIGS. 4A and 4B, andanother portion of the front section 156 a may be arranged directlyabove the side cavity portion 116 s ₂, as illustrated in FIGS. 4A and4C. The end section 156 c of the assisting waveguide 156 may be arrangeddirectly over the side cavity portion 116 s ₂ in the substrate 104.

The assisting waveguides 152, 158 may be arranged at an offset from, anddo not overlie, the cavity 116, as illustrated in FIGS. 3A, 3B, and 4D.Alternatively, the assisting waveguides may be at least partiallyarranged directly above the cavity in the base substrate 108, eventhough this embodiment is not illustrated in the accompanying drawings.

The assisting waveguides 152, 154, 156, 158 may include a material thathas a similar material composition as the underlying assistingwaveguides 136, 138. For example, the assisting waveguides 152, 154,156, 158 may include a material having a substantially similarrefractive index as the assisting waveguides 136, 138. The assistingwaveguides 152, 154, 156, 158 may be deposited using a depositiontechnique, including chemical vapor deposition process, and subsequentlypatterned using a patterning technique, including lithography andetching processes, during back-end-of-line processing. In anotherexample, the assisting waveguides 152, 154, 156, 158 may include adielectric material. In an embodiment of the disclosure, the assistingwaveguides 152, 154, 156, 158 may include silicon nitride. In anotherembodiment of the disclosure, the assisting waveguides 152, 154, 156,158 may include silicon carbide.

FIGS. 5A through 5C are exemplary top-down views of the edge coupler 100at a fabrication stage subsequent to FIG. 3A, according to embodimentsof the disclosure. FIGS. 6A through 6D are cross-sectional views of theedge coupler 100 taken generally along respective lines 4A-4A, 4B-4B,4C-4C, and 4D-4D in FIG. 3A, according to embodiments of the disclosure.A dielectric layer 164 may be arranged over the dielectric stack 148 andthe assisting waveguides 152, 154, 156, 158.

The dielectric layer 164 may include a side edge 166, and the side edge166 of the dielectric layer 164 may form a part of the side edge 102 ofthe edge coupler 100. The side edge 166 of the dielectric layer 164 maybe further arranged over the respective side edges 150, 146, 128, 110 ofthe dielectric stacks 148, 144, the dielectric layer 126, and the buriedinsulator layer 106. In an embodiment of the disclosure, the side edge166 of the dielectric layer 164 may be substantially planar. In anotherembodiment of the disclosure, the side edge 166 of the dielectric layer164 may be substantially coplanar with the respective side edges 150,146, 128, 110 of the dielectric stacks 148, 144, the dielectric layer126, and the buried insulator layer 106.

The dielectric layer 164 may include a dielectric material, for example,silicon dioxide and/or a dielectric material having a dielectricconstant lower than that of silicon dioxide. The dielectric layer 164may be deposited using a deposition technique, including a chemicalvapor deposition process, during back-of-line processing.

Assisting waveguides 168, 170, 172 may be arranged on the dielectriclayer 164 with a spaced arrangement. The assisting waveguide 170 may bearranged laterally between the assisting waveguides 168, 172, and mayhave a longitudinal axis 170L; the longitudinal axis 170L may besubstantially parallel to the longitudinal axis 100L of the edge coupler100. Each of the assisting waveguides 168, 170, 172 may have a finitelength and may be terminated by an end surface 174 and an end surface176 arranged laterally opposite the end surface 174.

The end surfaces 174 of the assisting waveguides 168, 170, 172 may bearranged over respective the side edges 166, 150, 146, 128, 110 of thedielectric layer 164, dielectric stack 148, the dielectric stack 144,the dielectric layer 126, and the buried insulator layer 106, and mayform a part of the side edge 102 of the edge coupler 100. In anembodiment of the disclosure, the end surfaces 174 of the assistingwaveguides 168, 170, 172 may be substantially planar. In anotherembodiment of the disclosure, the end surfaces 174 of the assistingwaveguides 168, 170, 172 may be substantially coplanar with therespective side edges 166, 150, 146, 128, 110 of the dielectric layer164, the dielectric stacks 148, 144, the dielectric layer 126, and theburied insulator layer 106. In yet another embodiment of the disclosure,the end surfaces 174 of the assisting waveguides 168, 170, 172 may besubstantially coplanar with the end surfaces of the assisting waveguides154, 156.

The assisting waveguides 168, 170, 172 may be arranged at an offset overthe underlying assisting waveguides 136, 138, 152, 154, 156, 158 and thewaveguide core 130, as illustrated in FIGS. 5A and 5C; the outlines ofthe assisting waveguides 152, 154, 156, 158, and the outlines of theassisting waveguides 136, 138, and the waveguide core 130 arediagrammatically shown as dash-dot lines and dash lines, respectively.

As illustrated in FIG. 5A, the assisting waveguides 168, 170, 172 may bearranged substantially parallel to each other and may have asubstantially equal length dimension. In an embodiment of thedisclosure, the end surfaces 174, 176 of the assisting waveguides 168,170, 172 may be substantially planar and may be substantially parallelto each other. In another embodiment of the disclosure, the end surfaces174 of the assisting waveguides 168, 170, 172 may be substantiallycoplanar with each other. In yet another embodiment of the disclosure,the assisting waveguides 168, 170, 172 may have a length shorter thanthe respective lengths of the assisting waveguides 154, 156.

Alternatively, as illustrated in FIG. 5B, the assisting waveguide 170may have an overlapping arrangement with the waveguide core 130. Similarto FIG. 5A, the end surfaces 174 of the assisting waveguides 168, 170,172 may be substantially coplanar with each other. In an embodiment ofthe disclosure, the assisting waveguide 170 may have a length that islonger than the respective lengths of the assisting waveguides 168, 172.In another embodiment of the disclosure, the end surfaces 176 of theassisting waveguides 168, 172 may be substantially coplanar with eachother, while the end surface 176 of the assisting waveguide 170 may bearranged at an offset therefrom. In yet another embodiment of thedisclosure, the assisting waveguides 168, 170, 172 may have a lengthshorter than the respective lengths of the assisting waveguides 154,156.

Additionally, as illustrated in FIG. 5C, the assisting waveguides 168,172 may be shaped as a bend that curves away from the longitudinal axis100L of the edge coupler 100 along the transverse axis 100T, similar tothe assisting waveguides 152, 158 in FIG. 3B. For example, a separationbetween the assisting waveguides 168, 170 may increase with increasingdistance from their respective end surfaces 174, and the assistingwaveguides 172 may curve away in an opposite direction relative to theassisting waveguide 168. In another embodiment of the disclosure, theassisting waveguides 168, 170, 172 may have substantially equal lengths.

The assisting waveguides 168, 170, 172 may have a width dimension thatvaries with position along its length. For example, the assistingwaveguides 168, 170, 172 may have a maximum width at the end surface174, and the assisting waveguides 168, 170, 172 may taper a directiontowards the end surface 176. In an embodiment of the disclosure, thewidth of the assisting waveguides 168, 170, 172 at the end surfaces 176may be narrower than the width at the end surface 174.

The assisting waveguides 168, 170, 172 may include a material that has asimilar material composition as the underlying assisting waveguides 136,138, 152, 154, 156, 158. For example, the assisting waveguides 168, 170,172 may include a dielectric material having a substantially similarrefractive index as the underlying assisting waveguides 136, 138, 152,154, 156, 158. In an embodiment of the disclosure, the assistingwaveguides 168, 170, 172 may include silicon nitride. In anotherembodiment of the disclosure, the assisting waveguides 168, 170, 172 mayinclude silicon carbide.

The assisting waveguides 168, 170, 172 may be deposited using adeposition technique, including a chemical vapor deposition process, andsubsequently patterned using a patterning technique, includinglithography and etching processes, during back-end-of-line processing.

Processing continues with the formation of an additional dielectriclayer 178 over the dielectric layer 164 and the assisting waveguides168, 170, 172. The dielectric layer 178 may include a side edge 180 thatmay be substantially coplanar with the respective side edges 166, 150,146, 128, 110 of the dielectric layer 164, the dielectric stacks 148,144, the dielectric layer 126, and the buried insulator layer 106. Thedielectric stack 148, the dielectric layer 164, and the dielectric layer178 may form a back-end-of-line stack 182 of the edge coupler 100.

The respective side edges 180, 166, 150 of the dielectric layer 178, thedielectric layer 164, the dielectric stack 148, and the end surfaces174, 160 of the assisting waveguides 168, 170, 172, and the assistingwaveguides 154, 154 may form a side edge 184 of the back-end-of-linestack 182. The side edge 184 of the back-end-of-line stack 182 isfurther a part of the side edge 102 of the edge coupler 100.

As presented in the above disclosure, edge couplers and methods offorming the same are presented. The edge couplers may include an arrayof waveguides arranged in the middle-of-line region and theback-end-of-line region of a photonic chip. For example, the waveguidecore 130 and the assisting waveguides 136, 138 may be arranged in themiddle-of-line region, and the assisting waveguides 152, 154, 156, 158,168, 170, 172 may be arranged in the back-end-of-line region.

The respective side edges 150, 166, 180 of the dielectric stack 148, thedielectric layer 164, and the dielectric layer 178, and respective endsurfaces 160, 174 of the assisting waveguides 152, 154, 156, 158, 168,170, 172 may be substantially coplanar with each other to form the sideedge 184 of the back-end-of-line stack 182.

One or more layers of dielectric materials, for example, a moisturebarrier layer, may be optionally arranged adjacent to the side edge 102of the edge coupler 100, to serve as a protection layer for the edgecoupler against environmental effects.

During an assembly phase, a light source, for example, an optical fibermay be laterally arranged adjacent to the side edge 184 ofback-end-of-line stack 182. Due to the placements of the assistingwaveguides 152, 154, 156, 158, 168, 170, 172 in the back-end-of-linestack 182, the optical fiber may be arranged adjacent to the side edge102 of the edge coupler 100, and the optical fiber may be placed overthe base substrate 108 or in a groove or an opening formed in the basesubstrate 108.

The edge coupler 100 may be a multi-stage edge coupler for the couplingof optical signals from the light source to the waveguide core 130. Forexample, the edge coupler 100 may include multi-stage cascadingassisting waveguides along with the vertical axis 100V of the edgecoupler 100, such as the assisting waveguides 168, 170, 175 at an upperlevel, the assisting waveguides 152, 154, 156, 158 at a lower level, andthe assisting waveguides 136, 138 and the waveguide core 130 at thelowest level.

The optical signals from the light source may propagate into the edgecoupler 100 through the assisting waveguides 168, 170, 175 at the upperlevel and the assisting waveguides 154, 156 at the lower level. As theassisting waveguides 168, 170, 175 may have a taper structure and may beunable to fully confine the mode of the optical signals, the mode of theoptical signals may be pushed to the assisting waveguides 154, 156 atthe lower level that has an inverse taper structure that can betterconfine the mode of the optical signals.

The optical signals may continue to propagate through the assistingwaveguides 154, 156, and when the optical signals approach the endsections of the assisting waveguides 154, 156 that have a taperstructure, the mode of the optical signals may again be pushed to alower level and into the waveguide core 130. The optical signals thencontinue to propagate through the waveguide core 130 and out of the edgecoupler 100 to the other components in the photonic chip.

As the mode of the optical signals becomes closer to the substrate 104,there may be potential optical signal losses through the substrate 104,adversely affecting the performance of the edge coupler 100. The sealedcavity 116 in the substrate advantageously minimizes optical signallosses through the substrate 104. The end section 130 c of the waveguidecore 130 may be arranged at an offset from the sealed cavity 116 tobetter confine the mode of the optical signals within the waveguide core130. The sealed cavity 116 being fully confined within the substrate 104also improves the manufacturability of edge coupler 100.

The array of assisting waveguides 152, 154, 156, 158, 168, 170, 172, andthe dielectric stack 148, the dielectric layer 164, and the dielectriclayer 178 in the different levels of the back-end-of-line stack 182 mayminimize diffraction or reflection light at the wavelength of operation,and therefore, act as an effective optical material and form ametamaterial.

The terms “top”, “bottom”, “over”, “under”, and the like in thedescription and the claims, if any, are used for descriptive purposesand not necessarily for describing permanent relative positions. It isto be understood that the terms so used are interchangeable underappropriate circumstances such that the embodiments of the devicesdescribed herein are, for example, capable of operation in otherorientations than those illustrated or otherwise described herein.

Additionally, the formation of a first feature over or on a secondfeature in the description that follows may include embodiments in whichthe first and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

Similarly, if a method is described herein as involving a series ofsteps, the order of such steps as presented herein is not necessarilythe only order in which such steps may be performed, and certain of thestated steps may possibly be omitted and/or certain other steps notdescribed herein may possibly be added to the method. Furthermore, theterms “comprise”, “include”, “have”, and any variations thereof, areintended to cover a non-exclusive inclusion, such that a process,method, article, or device that comprises a list of features is notnecessarily limited to those features but may include other features notexpressly listed or inherent to such process, method, article, ordevice. Occurrences of the phrase “in an embodiment” herein do notnecessarily all refer to the same embodiment.

In addition, unless otherwise indicated, all numbers expressingquantities, ratios, and numerical properties of materials, reactionconditions, and so forth used in the specification and claims are to beunderstood as being modified in all instances by the term “about”.

Furthermore, approximating language, as used herein throughout thespecification and claims, may be applied to modify any quantitativerepresentation that could permissibly vary without resulting in a changein the basic function to which it is related. Accordingly, a valuemodified by a term or terms, such as “approximately”, “about,”,“substantially” is not limited to the precise value specified. In someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. In other instances, theapproximating language may correspond to within normal tolerances of thesemiconductor industry. For example, “substantially coplanar” meanssubstantially in a same plane within normal tolerances of thesemiconductor industry, and “substantially perpendicular” means at anangle of 90 degrees plus or minus a normal tolerance of thesemiconductor industry.

While several exemplary embodiments have been presented in the abovedetailed description of the device, it should be appreciated that anumber of variations exist. It should further be appreciated that theembodiments are only examples, and are not intended to limit the scope,applicability, dimensions, or configuration of the device in any way.Rather, the above detailed description will provide those skilled in theart with a convenient road map for implementing an exemplary embodimentof the device, it being understood that various changes may be made inthe function and arrangement of features and methods of fabricationdescribed in an exemplary embodiment without departing from the scope ofthis disclosure as set forth in the appended claims.

What is claimed is:
 1. An edge coupler, comprising: a substrate having atop surface; a sealed cavity in the substrate, the sealed cavity havingvarying depths relative to the top surface of the substrate; a waveguidecore over the sealed cavity; and a back-end-of-line stack over thewaveguide core, the back-end-of-line stack comprises: a side edge; aninterlayer dielectric layer; and an assisting waveguide on theinterlayer dielectric layer adjacent to the side edge, and the assistingwaveguide and the waveguide core have an overlapping arrangement withthe sealed cavity in the substrate.
 2. The edge coupler of claim 1,wherein the sealed cavity comprises: a first region proximate to theside edge of the back-end-of-line stack; and a second region adjacent tothe first region, wherein the first region has a depth, the secondregion has a depth, and the depth of the second region is deeper thanthe depth of the first region.
 3. The edge coupler of claim 2, whereinthe waveguide core comprises a front section, a middle section, and anend section, and the front section is arranged directly above the firstregion of the sealed cavity.
 4. The edge coupler of claim 2, wherein thewaveguide core comprises a front section, a middle section, and an endsection, wherein a portion of the middle section is arranged directlyover the second region of the sealed cavity.
 5. The edge coupler ofclaim 4, wherein the middle section of the waveguide core is taperedtowards the front section thereof.
 6. The edge coupler of claim 2,wherein the waveguide core comprises a front section, a middle section,and an end section, wherein the end section is arranged at an offsetfrom the sealed cavity.
 7. The edge coupler of claim 1, wherein thewaveguide core is centered above the sealed cavity.
 8. The edge couplerof claim 1, wherein the waveguide core has an end surface, and the endsurface of the waveguide core is spaced apart from the side edge of theback-end-of-line stack.
 9. The edge coupler of claim 1, wherein thewaveguide core has a length, and the sealed cavity has a length that issubstantially parallel to the length of the waveguide core.
 10. The edgecoupler of claim 1, wherein the assisting waveguide has an end surface,and the end surface of the assisting waveguide is substantially coplanarwith the side edge of the back-end-of-line stack.
 11. The edge couplerof claim 1, wherein the substrate further comprises comprising aninsulator layer over the substrate, and the sealed cavity iscollectively surrounded by the insulator layer and the substrate.
 12. Anedge coupler, comprising: a substrate having a bottom surface, thesubstrate having varying heights relative to the bottom surface of thesubstrate; a sealed cavity in the substrate; a waveguide core over thesubstrate; and a back-end-of-line stack over the waveguide core, theback-end-of-line stack comprises: a side edge; an interlayer dielectriclayer; and a first assisting waveguide on the interlayer dielectriclayer adjacent to the side edge, and the first assisting waveguide andthe waveguide core have an overlapping arrangement with the sealedcavity in the substrate.
 13. The edge coupler of claim 12, wherein thesealed cavity comprises: a first region proximate to theback-end-of-line stack; and a second region adjacent to the firstregion, wherein the substrate has a first height under the first regionof the sealed cavity, a second height under the second region of thesealed cavity, and the second height of the substrate is shorter thanthe first height.
 14. The edge coupler of claim 13, wherein the firstregion of the sealed cavity has a length, the second region of thesealed cavity has a length, and the length of the second region islonger than the length of the first region.
 15. The edge coupler ofclaim 12, further comprising a second assisting waveguide, the secondassisting waveguide is on the interlayer dielectric layer at the samelevel as the first assisting waveguide, wherein the second assistingwaveguide is arranged at an offset from the sealed cavity in thesubstrate.
 16. The edge coupler of claim 15, wherein the first assistingwaveguide has an end surface that is substantially coplanar with theside edge of the back-end-of-line stack, and the second assistingwaveguide has a side edge proximate to and spaced apart from the sideedge of the back-end-of-line stack.
 17. The edge coupler of claim 16,wherein the first assisting waveguide has a length, the second assistingwaveguide has a length, and the length of the second assisting waveguideis shorter than the length of the first assisting waveguide.
 18. Theedge coupler of claim 16, wherein the second assisting waveguide has asubstantially constant width dimension, and the first assistingwaveguide has a varying width dimension.
 19. A method of forming an edgecoupler, comprising: forming a sealed cavity in a substrate, the cavityis collectively surrounded by an insulator layer and a base substrate ofthe substrate; forming a waveguide core over the sealed cavity; andforming a back-end-of-line stack over the waveguide core, whereinforming the back-end-of-line stack comprises: forming an interlayerdielectric layer over the waveguide core; and forming an assistingwaveguide over the interlayer dielectric layer, wherein a side edge ofthe interlayer dielectric layer and an end surface of the assistingwaveguide form a part of a side edge of the back-end-of-line stack, andthe assisting waveguide and the waveguide core have an overlappingarrangement with the sealed cavity in the substrate.
 20. The method ofclaim 19, wherein forming the sealed cavity in the substrate comprises:patterning a plurality of openings through the insulator layer to thebase substrate; and etching the base substrate with an isotropic etchingprocess to form the cavity under the insulator layer.